How to avoid require avalanches with node.js? - node.js

Question

I want to cache some data in node server:
rpc data provider like this:
var cnt = 0;
function rpcDataProvider(areaId) {
return Q.delay(100).then(() => {
cnt += 1
console.log("I am rpcDataProvider and I am not so fast. I am requested " + cnt + " times.");
const base = areaId * 10;
var arr = [];
for (var i = 0; i < 10; i ++) {
arr.push(base + i);
}
return arr;
})
}
http server
// cache data
var provinceList;
var cityList;
var countyList;
function getProvinceList() {
if (provinceList && provinceList.length > 0) return Q.resolve(provinceList);
return rpcDataProvider(1).then((rv) => {
provinceList = rv;
return rv;
});
}
function getCityList() {
if (cityList && cityList.length > 0) return Q.resolve(cityList);
return getProvinceList().then((provinceList) => {
return Q.all(provinceList.map((item) => rpcDataProvider(item))).then(rvs => {
cityList = rvs.reduce((prev, cur) => prev.concat(cur));
return cityList;
});
});
}
function getCountyList() {
if (countyList && countyList.length > 0) return Q.resolve(countyList);
return getCityList().then((cityList) => {
return Q.all(cityList.map((item) => rpcDataProvider(item))).then(rvs => {
countyList = rvs.reduce((prev, cur) => prev.concat(cur));
return countyList;
})
})
}
function api1() {
console.log("I am http api1");
return getProvinceList();
}
function api2() {
console.log("I am http api2");
return getCityList();
}
function api3() {
console.log("I am http api3");
return getCountyList();
}
function api4() {
console.log("I am http api4");
return getCountyList();
}
function api5() {
console.log("I am http api5");
return getCountyList();
}
client request:
function httpRequest() {
console.log("I am client.");
Q.all([api1(), api2(), api3(), api4(), api5()]);
}
httpRequest();
the problem is that parallel requests, the cache doest not work!

Answer 1

Don't cache the promise value. Cache the promise.
// cache data
var provinceList;
var cityList;
var countyList;
function rpcReduceAndConcat(list) {
return Q.all(list.map(rpcDataProvider)).then(rvs => {
return rvs.reduce((prev, cur) => prev.concat(cur));
});
}
function getProvinceList() {
if (!provinceList) provinceList = rpcDataProvider(1);
return provinceList;
}
function getCityList() {
if (!cityList) cityList = getProvinceList().then(rpcReduceAndConcat);
return cityList;
}
function getCountyList() {
if (!countyList) countyList = getCityList().then(rpcReduceAndConcat);
return countyList;
}